With recent trends toward portable and wireless electronic devices, there is increasing demand for small and lightweight nonaqueous electrolyte secondary batteries having high energy density, such as lithium ion secondary batteries, as a power source for these electronic devices.
In general, a nonaqueous electrolyte secondary battery includes a positive electrode, a negative electrode, a separator and a nonaqueous electrolyte. Each of the positive and negative electrodes is generally formed of a current collector and a material mixture layer containing an active material and a binder formed on the current collector. As the active material of the positive electrode, lithium composite oxides capable of reversibly causing an electrochemical reaction with lithium may be used. As the active material of the negative electrode, lithium metals, lithium alloys, carbons capable of inserting and extracting lithium ions may be used. The separator may be a film capable of supporting the nonaqueous electrolyte and electrically insulating the positive and negative electrodes. The nonaqueous electrolyte may be an aprotic organic solvent dissolving therein lithium salt such as LiClO4, LiPF6 or the like. The positive and negative electrodes are stacked or wound with the separator interposed therebetween to form an electrode group. The electrode group is placed in a battery case together with the nonaqueous electrolyte and the case is sealed with a lid. In this way, the nonaqueous electrolyte secondary battery is manufactured.
The nonaqueous electrolyte secondary battery easily generates heat when it is overcharged or an internal short circuit occurs in it. When the battery in a charged state is exposed to a high temperature environment, active oxygen is eliminated from the positive electrode active material (in particular, lithium composite oxide) and reacts with the nonaqueous electrolyte. Heat generated by the reaction accelerates the oxygen elimination from the positive electrode active material. This chain reaction is considered as a main cause of the heat generation of the battery. As the chain reaction continues, the separator is molten or shrunk to cause an internal short circuit between the positive and negative electrodes (or an internal short-circuited part is enlarged). As a result, large current flows between the electrodes, and therefore the battery is overheated and becomes instable.
In order to avoid this drawback, Patent Literature 1 discloses a method for suppressing the heat generation caused by the short circuit by increasing electrical resistance of the active material of the electrode. More specifically, lithium-cobalt composite oxide which shows a resistance coefficient of not less than 1 mΩ·cm and not more than 40 mΩ·cm when filling density thereof is 3.8 g/cm3 is used as the positive electrode active material to suppress the heat generation in the short circuit.
However, according to this method, as the positive electrode active material is increased in resistance, the internal resistance is also increased. As a result, satisfactory power cannot be output, for example, in the case of a high power secondary battery generally used as a power source for driving an electric vehicle.
Patent Literature 2 discloses a method for suppressing the heat generation due to the short circuit with less increase in internal resistance. According to this method, a resistance layer having higher resistance than that of a current collector is formed on the surface of the current collector and an active material layer is formed on the resistance layer. More specifically, the resistance layer having a resistance value of 0.1-100 Ω·cm2 is formed so as to prevent the large current flow even if an internal short circuit occurs.    [Patent Literature 1] Publication of Japanese Patent Application No. 2001-297763    [Patent Literature 2] Publication of Japanese Patent Application No 10-199574